The control of industrial processes and, in particular, the continuous non-contacting evaluation of material during manufacture and production is a subject of continued innovation. Given that essentially all such processes call for some form of guidance control of material, investigators direct much attention toward developing improved monitoring and measuring systems.
One obstacle generally facing all monitoring systems is that materials undergoing these production procedures vary widely in chemical composition, make-up, density, and the like, for example, from transparent plastics to steel billets heated to incandescence. Additionally, speed of movement of these materials ranges from the barely discernible to very fast. Temper mills in the steel industry, for instance, often will convey sheet steel at speeds of about 90 mph (144 km/hr). As another consideration, many industries call for extremely precise and accurate measurement capabilities. As an example, many industries that use steel require that the initial production of rolled steel exhibit accurately controlled widthwise or lengthwise dimensions.
To meet industry demands and achieve the necessary dimensional control monitoring, accurate measurements generally must be carried out on a non-contacting and substantially continuous basis. The resultant control process not only achieves production accuracy, but also minimizes material waste otherwise resulting from continued production under out of tolerance conditions.
Presently, there are a variety of techniques available to monitor, track, and measure moving material. The invention described in U.S. Pat. No. 5,220,177 by Harris, issued Jun. 15, 1993, entitled "Method and Apparatus for Edge Detection and Location" (expressly incorporated herein by reference), has met with market acceptance as a non-contacting edge detector. The Harris invention employs a linear array of light emitting diodes (LEDs) positioned on one side of the material to be monitored wherein each diode of the array is energized to emit light exhibiting substantially uniform intensity. Positioned above the moving material and opposite the associated diode array is a tuned photoresponsive receiver which reacts to the illumination emanating from those dimes which are unblocked or partially blocked at the edges of the moving material. The receiver and associated control system are called upon to carry out an extrapolation process to develop mandated accuracy in locating the position of an edge. This extrapolation is based upon the observation that each LED in the emitting array produces a cone of light, and the light cones from adjacent LED's overlap each other in the light path to the photoresponsive receiver. An edge of a product being measured blocking the light path from the emitting diodes to the receiver will attenuate the light from more than one diode. The signal processing procedure carrying out extrapolation takes samples of the peak amplitude of the light received in sequence from the partially blocked and unblocked LEDs and develops therefrom a time-based stairstep light output pattern representing a scan across the material edge which, in effect, is smoothed through the utilization of a low pass filtering stage. The edge position of the material being observed, then, is defined as the time equivalent point on this smooth curve signal where the voltage drops to one-half of the peak LED signal amplitude.
A number of other systems which incorporate both an optical source be it an LED or a laser, and a photodetector are readily apparent. These systems may use either single or multiple sources and/or detectors for measuring or sensing some feature in the optical path between the source and detector. This feature may be a flaw, hole or edge of some material being passed through the optical path. Another possibility may be strictly sensing presence of absence of the source radiation. In these cases, the system may be used for position sensing of either the source or detector components. A third system would be one which detects and/or measures the concentration of airborne particles or gases which may absorb the source radiation. Generally, transmitted light is detected and converted into an electrical signal. The signal may then be amplified and compared against a threshold. In other applications the magnitude of the electrical signal is an indicia of the state of the optical path, i.e., whether it is blocked, or something is partially absorbing the source radiation. Other embodiments operate similarly in that they use a series of conventional charge coupled devices (CCDs) oppositely disposed from an infrared light source. As light falls on the CCD photosensitive elements, signals are produced and converted to represent the intensity pattern. Thereafter, a transition point between a high-voltage value and a low-voltage value is used to represent the location of the edge.
These devices, however, have proven less than effective in certain adverse ambient conditions. Specifically, in many manufacture and production industries, air borne particles, such as dust, dirt, smoke, pollutants, and the like, become prevalent in and around the detecting system. As a result, these particles tend to accumulate on the detector and emitter obscuring readings and signal detection capabilities.
Other inherent shortcomings with past devices include, for example, variations in the intensity of each detector's output is inevitable due to changes in the sensitivity of the detector. Such variations typically stem from changes in the supply voltage, detector aging, and general wear on the optical components. Particularly in harsh industrial environments, component lifetimes are greatly reduced. For example, LED detectors in mills often are required to measure steel which typically is heated to a level of incandescence above 3,000.degree. F. (1,649.degree. C). Additionally, depending on the industrial application, from 10 to 400 diodes may be needed. It has been observed, however, that LED light output from an average purchased lot varies by 30% or more. Lastly, many of these devices are expensive to manufacture in that they involve an overall complex system necessarily requiting a significant amount of electronics.
Semiconductor based devices are also used as detectors. These detectors generally are used to detect light and radiation, particularly high level radiation such as .gamma.-ray or x-ray radiation, or longer radiation such as infrared. Conventional high level detectors typically have a single-crystal silicon based substrate with a p-n junction or Schottky junction. A reverse bias is applied across the junction to separate substrate surface regions to form a depletion region generally equal in size to the range of mobility of generated electrons. As radiation penetrates and strikes the depletion region, electron-hole pairs (EHPs) are created to provide a detectable signal.
Another class of semiconductor-based radiation detectors employed in industry are photovoltaic detectors which include photodiodes and junction diode detectors. In photodiode systems, a detectable current resulting from drift of minority carriers across a p-n junction occurs as a result of thermal or optical excitation of EHPs. In particular, radiation having h.nu. greater than the band gap energy of the semiconductor incident on the junction will generate an EHP. Correspondingly, a drift current is generated such that EHPs generally within a diffusion length of each side of the junction diffuse to the transition region, thereafter providing a detectable signal. This phenomenon is characteristic of photodiodes operating in the third or fourth quadants of its I-V characteristics curve.
Presently, these semiconductor-based radiation detectors are not known to be successfully employed with positional and dimensional monitoring instrumentation. One problem associated with these devices is the unwanted detection of noise and other extraneous radiation. Specifically, all incident radiation having energy greater than the semiconductor band gap energy causes the generation of an EHP. Thus, for example, if photovoltaic detectors are being used to detect infrared radiation, any other radiation with a shorter wavelength similarly would excite the device, thus causing unwanted EHP generation. Problems particularly arise in environments exhibiting a wide range of ambient radiation. Steel mills, for example, have radiating sources ranging at least from molten steel to overhead strobes on motorized tow-motors.
For a fuller understanding of this technology including examples of semiconductor detectors, refer to the following: B. G. Streetman, Solid State Electronic Devices, (1980); U.S. Pat. No. 4,210,805, entitled "Semiconductor Radiation Detector", issued Jul. 1, 1980; U.S. Pat. No. 4,679,063, entitled "Infra Red Detectors", issued Jul. 7, 1987; U.S. Pat. No. 4,896,200, entitled "Novel Semiconductor-Based Radiation Detector", issued Jan. 23, 1990; and U.S. Pat. No. 5,019,886, entitled "Semiconductor-Based Radiation-Detector Element", issued May 28, 1991, the disclosures of which are expressly incorporated herein by reference.